A spread spectrum communication (SSC) system has such advantages, among others, that it is resistive against interference and fading and that it has a secrecy. However, the system still involves a lot of technical problems one of which is how to reliably synchronize a PN code contained in a received signal with a reference code prepared in a receiver. A system using a convolver as a correlator is effective for overcoming the problem. An example thereof is shown in FIGS. 9 through 11.
FIG. 9 shows an arrangement on the part of a transmitter. Reference numeral 23 refers to a clock generator, 24 to a PN code generator, 25 to a carrier oscillator, 26 to a mixer, 27 to a band pass filter, 28 to an amplifier, and 29 to a transmission antenna.
FIG. 11 shows an arrangement on the part of a receiver. Reference numeral 30 refers to a reception antenna, 31 to a band pass filter, 32 to an amplifier, 33 to a local oscillator, 34 to a mixer, 35 to an AGC circuit, 36 to a band pass filter, 38 to a clock generator, 39 to an oscillator, 40 to a mixer, 41 to a surface acoustic wave (SAW) convolver, 42 to a band pass filter, 43 to an amplifier, 44 to an envelope detector, 45 to a comparator, 48 to a comparing demodulator, 49 to a control circuit, and 50 to a PN code generator.
As shown in FIG. 10, the receiver switches the PN code between PN1 and PN2 having different code patterns in response to information data, and performs CSK (Code Shift Keying) modulation.
The receiver, which demodulates the transmitted data by applying a received signal and a reference signal to the SAW convolvers 41, improves the process gain by tracking the PN code contained in the received signal through a code tracking loop CTL.
The conventional system, however, is readily affected by a noise under a poor S/N ratio of the received signal, which results in an unstable tracking operation of the code tracking loop which, in turn, causes an increase of the bit error in the received data.